Luminous body for generating white light

ABSTRACT

A luminous body for generating white light is described which is provided with a combination of a light-emitting diode radiating blue light and a fluorescent lamp comprising green and red phosphors.

The invention relates to a luminous body for generating white lighthaving the particular features of an enhanced useful life and a greatercolor point stability.

It is known that a three-color phosphor mixture is used in high-qualityfluorescent lamps for the generation of white light. The color point ofthe lamp is then determined by the mixing ratio of the phosphors.

The blue phosphors used at present represent a particular problem herefor maintaining the color point because of the sensitivity of theactivator Eu²⁺. As a result, fluorescent lamps exhibit an undesirableshift in their color point during lamp operation, which is particularlyunpleasant in the case of compact fluorescent lamps (energy-savinglamps).

Fluorescent lamps are known from Japanese patent application JP-10275600A whose light spectrum can be modified by a light-emitting diode. Thelight-emitting diode, however, is integrated in the burner in this case,so that it comes into contact with the discharge. A gas discharge is ahighly aggressive medium which would quickly destroy the light-emittingdiode. In addition, the complete omission of the blue phosphor, whichhas always been used until now, is not proposed therein.

It is accordingly an object to develop luminous bodies which areremarkable for a long useful life and an improved color point stability.

This object is achieved by means of a luminous body which, for thepurpose of generating white light, is provided with a combination oflight-emitting diodes generating blue light (380-500 nm) and one orseveral fluorescent lamps comprising green and red phosphors. Thefluorescent lamps emit a yellowish-white light with a color temperatureof between 2500 and 3000 K owing to the absence of the blue emission.

The light-emitting diodes providing blue light may be arranged in theluminous body according to the invention in various manners, as long asa good light mixing and a homogeneous light are achieved thereby.However, they cannot be directly integrated into the burner of afluorescent lamp, since the contact with the discharge wouldconsiderably reduce their useful lives.

Since the useful life of the blue light-emitting diode is much longerthan that of conventional gas discharge sources, the useful life and thecolor point stability of the luminous body are considerably enhanced bythe invention.

The short life of conventional luminous bodies is mainly due to the bluelight-emitting phosphors containing Eu²⁺ such as BaMgAl₁₀O₁₇:Eu andSr₅(PO₄)₃(F,Cl):Eu. These phosphors have a comparatively fastdepreciation and have given rise to numerous experiments aimed atimproving the quality of the phosphors so as to avoid their fastdepreciation. Alternative phosphors for replacing these bluelight-emitting substances, however, have not been found until now.

The emission of blue light is a decisive element in a luminous bodyradiating white light, in particular at high color temperatures. Sincethe phosphor radiating blue light forms the weak spot of all fluorescentlamps (TL, PL and CFL) used at present, the solution according to theinvention considerably reduces this problem of lamp life for theselamps, which is of essential importance for the economy of such lamps.

The luminous body according to the invention is preferably formed by acombination of a conventional fluorescent lamp provided with red andgreen phosphors and an InGaN or AlInGaV light-emitting diode providingblue light in a wavelength range of between 380 and 500 nm. The twolight sources, i.e. the fluorescent lamp(s) and the blue light-emittingdiodes, are accommodated in a single lamp housing or in the sameluminaire.

The invention will be clarified with reference to the drawing comprisingFIGS. 10, 1 to 6, in which

FIG. 10 shows a lamp according to the invention with light-emittingdiodes providing blue light,

FIG. 1 shows a rectangular light tile with light-emitting diodesproviding blue light and with fluorescent tubular lamps,

FIG. 2 shows a round light tile with light-emitting diodes providingblue light and a circular tubular fluorescent lamp,

FIG. 3 shows the emission spectrum of a fluorescent lamp withLaMgAl₁₁O₁₉:CeTb and Y₂O₃:Eu,

FIG. 4 shows the emission spectrum of a light-emitting diode thatradiates blue light,

FIG. 5 shows the color points of a light-emitting diode that radiatesblue light, a fluorescent lamp, and the light source formed by theformer two at 5000 K CCT, and FIG. 6 shows the emission spectrum of aluminous body combining light-emitting diodes radiating blue light andfluorescent lamps provided with LaMgAl₁₁O₁₉:CeTb and Y₂O₃:Eu, at 5000 KCCT.

FIG. 10 shows a luminous body according to the invention with thelight-emitting diodes 1 radiating blue light, the fluorescent lamp 2,and the outer lamp bulb which is coated with a light-scattering layer.It is possible here, for example, for three InGaN light-emitting diodesradiating blue light with an emission maximum of 480 nm to beaccommodated inside the outer lamp bulb 3. The fluorescent lamp iscoated with LaMgAl₁₁O₁₉:CeTb (green) and Y₂O₃:Eu (red), such that ayellowish-white light is generated if the fluorescent lamp is operatedalone. The current supply to the light-emitting diodes is integratedinto the base of the luminous body. When the light-emitting diodes areswitched on together with the fluorescent lamp, the light will be mixedinside the lamp bulb and the overall radiated light will appear white,which corresponds to a color temperature T_(c)=5000 K. An increase inthe current through the light-emitting diodes leads to an increasedradiation of blue light, which causes the color temperature of theoverall radiated light to rise; a reduction in the current reduces thequantity of blue light and accordingly lowers the color temperature ofthe emitted light.

The color rendering index achieved in this manner lies above 80 for allcolor temperatures between 2600 and 10,000 K.

The luminous bodies according to the invention as shown in FIGS. 1 and 2have planar or circular shapes which are covered with PMMA plates. Thefluorescent lamps comprise a green phosphor, for example LaPO₄:CeTb,LaMgAl₁₁O₁₉:CeTb, or GdMgB₅O₁₀:CeTb, and, for example, Y₂O₃:Eu orY(V,P)O₄:Eu as the red phosphor.

FIG. 3 shows the emission spectrum of such a fluorescent lamp. The colorpoints of these fluorescent lamps (x=0.47, y=0.42) lie close to thecolor point of YAGaG:Ce (x=0.48, y=0.50) used in light-emitting diodesthat radiate white light (see FIG. 5).

An increase in the current strength renders it possible to raise thecolor temperature of the blue light-emitting diodes up to 10,000 K, withthe result that the color point lies close to the blackbody locus forthis color temperature, provided the blue light-emitting diode wascorrectly chosen. Most suitable are blue light-emitting diodes with acolor point at x=0.1 and y=0.2.

The advantages of the luminous body according to the invention lie in animproved luminous efficacy and a higher color point stability, becausethe useful life of the light-emitting diode is much longer than theuseful life of a conventional blue phosphor as used until now influorescent lamps.

In addition, the light color can be modified more easily in the luminousbody according to the invention in that simply the radiation of the bluelight of the light-emitting diode is varied through an increase ordecrease in the current strength. It is necessary for this that aseparate current supply is provided for the light-emitting diode, sothat the current flow to the light-emitting diode, and thus its lightemission, can be controlled independently of the light emission of thefluorescent lamp. This renders it possible to adjust the colortemperature of the blue light-emitting diode over a wide range, forexample between 2660 K and 10,000 K.

The combination of two light sources which should appear to be only asingle one necessitates certain optical means for mixing the light in asuitable manner. In their absence the light combination would be visibleand the radiated light would appear to be inhomogeneous. It isaccordingly necessary to integrate the light-emitting diodes into thelamp such that a good light distribution and mixing are safeguarded.

This may be readily achieved in GLS-look-alike CFL-I lamps because anouter thermoplastic bulb has already been developed for these, whichbulb scatters the light by means of a powder layer. The bluelight-emitting diodes then merely have to be accommodated inside theouter bulb of the CFL-I lamp, while the necessary electronic controlmeans can be accommodated in the lamp base. Alternative constructionsfor implementing the present invention are also conceivable, for exampleaccommodating the blue light-emitting diode inside the same lamphousing, or alternatively in a lamp housing separate from thefluorescent lamp. The light-emitting diode may also be used inconjunction with a PMMA plastic foil or plastic plate which is coveredwith a light-scattering layer at one side and with a structure forcoupling out the light on the other side.

A compact fluorescent lamp, a Hg low-pressure gas discharge lamp, a Hghigh-pressure gas discharge lamp, or a sulphur lamp may be used as thefluorescent lamp in the luminous body according to the invention. Theuseful life and the color point stability of the luminous body areconsiderably improved in the generation of white light in all cases.

1. A luminous body for generating white light, characterized in that itis provided with a combination of light-emitting diodes radiating bluelight and a fluorescent lamp comprising green and red phosphors.
 2. Aluminous body as claimed in claim 1, characterized in that thelight-emitting diode radiating blue light comprises a semiconductorbased on InGaN or AlInGaN.
 3. A luminous body as claimed in claims 1 and2, characterized in that the blue light radiated by the light-emittingdiode lies in a wavelength range of between 380 and 500 nm.
 4. Aluminous body as claimed in claims 1 to 3, characterized in that thefluorescent lamp is a compact fluorescent lamp (energy-saving lamp), aHg low-pressure gas discharge lamp (fluorescent tube), a Hghigh-pressure gas discharge lamp, or a sulphur lamp.
 5. A luminous bodyas claimed in claims 1 to 4, characterized in that the fluorescent lampcomprises at least one phosphor from the group of LaPO₄:CeTb,LaMgAl₁₁O₁₉:CeTb, GdMgB₅O₁₀:CeTb, Y₂O₃:Eu, Y(V,P)O₄:Eu, or one of themixtures thereof.
 6. A luminous body as claimed in claims 1 to 5,characterized in that the light-emitting diode radiating blue light isaccommodated inside the same lamp housing as the fluorescent lamp.
 7. Aluminous body as claimed in claims 1 to 8, characterized in that thelight-emitting diode radiating blue light is accommodated in a lamphousing separate from the fluorescent lamp.
 8. A luminous body asclaimed in claims 1 to 7, characterized in that a separate currentsupply is provided for the light-emitting diode radiating blue light,such that the current flow to the light-emitting diode and thus itslight emission can be controlled independently of the light emission ofthe fluorescent lamp.